Stegosaurus plate shape may reveal gender

Sexual dimorphismThe shape of the bony plates along the back of the long-extinct Stegosaurus may have differed if the dinosaur were male or female, researchers say.

The Stegosaurus was a lumbering, plant-eating dinosaur with two rows of back plates. They walked on four feet and roamed Earth about 150 million years ago.

Rounder plates were likely found on males and taller plates on females, said the study in the journal PLOS ONE.

In birds, more colourful feathers are often seen in males, such as the male peacock's towering display of tail feathers which are used to attract mates. Females tend to be duller in colour, with browns dominating.

Birds are related to dinosaurs, but these differences, known as sexual dimorphism, have been difficult to prove in extinct creatures.

"These stegosaurs seem to provide the first really convincing evidence for sexual dimorphism in a dinosaur species," says lead author Evan Saitta of the University of Bristol.

After studying Stegosaurus remains in Montana for years, Saitta began to hypothesize that the difference in the plates - with the broad ones being some 45 per cent bigger than the slender spiky ones - was not indicative of different species of Stegosaurus, but different sexes.

He ruled out other possibilities, such as the different shapes presenting varying stages of growth, after CT scans showed that the bone tissues had ceased growing in both varieties.

"As males typically invest more in their ornamentation, the larger, wide plates likely came from males. These broad plates would have provided a great display surface to attract mates," says Saitta.

"The tall plates might have functioned as prickly predator deterrents in females," he adds.

Michael Benton, professor of vertebrate palaeontology at the University of Bristol notes that Saitta made the discovery while he was completing his undergraduate thesis at Princeton University.

"It's very impressive when an undergraduate makes such a major scientific discovery," Benton says.